Re: [electricboats] Thoughts on 12 volt Lifepo4 batteries

All batteries, regardless of chemistry, in fact all sources of electrical power whatsoever, will have some 'equivalent series resistance' aka ESR. Effectively, resistance internal to the source itself through which load current must pass _before_ leaving the terminals.  Source resistance is another name for it.  You can easily find ESR with a voltmeter if you already know the current and can switch it on and off.

The change in terminal voltage between on and off divided by the current reveals the source's internal resistance.  Volts divided by amps gives us ohms...always.

On my '12V' '1800Ah' 100A system (2x50A motors,) the voltage went a good half a volt down at the 'terminals' when switched on.  0.5V divided by 100A is 0.005 Ohms.  Five milliohms equivalent resistance _inside_ the battery. Additionally, I lost another 0.4V thru my wiring: 20' of #000 plus 12' of #0 copper wire (X2 for send and return, naturally.)  Just like Ohm said I would.  It was kinda remarkable, actually, how exactly, in fact.

The '_problem_' with a large amp-hour capacity, the esr is often driven to a very low value. ESR of parallel cells combine in parallel and reduce the equivalent value, which at some point does _not_ drop much voltage, especially at low current.  In return, you get improved efficiency for the lower impedance source.  See: 'Maximum Power Transfer' in your electrical theory book for more details.  But maybe _don't_ see MinnKota for warranty service, I would think.  It's not their fault if you can't physics.

tldr; More powerful sources have low 'output resistance' and run more efficiently, but must be used more carefully.   

(Mr Mackey Voice)  Resistance is _bad_ m'kay... Er, but you might actually want some... :-)

IDK the failure mode for the MinnKotas tho, with over voltage... Maybe it's "yeah you really shouldn't but nothing remarkable really happens." Maybe it burns the brushes up?  Overheated windings melt the insulation and short turns?  Someone shout out if you find that answer...eh?  :-)

On Tue, May 3, 2022 at 1:52 PM, Skray775

<kelly@skraye.com> wrote:

On Tue, May 3, 2022 at 02:40 PM, Skray775 wrote:

Well that is a bit of bad news about the minnkota specs. I believe that takes the option of A123's off the table?

 

The current setup has a space on the left for a starting battery for the 150 outboard motor and a separate 5 amp charger for that battery. The three trolling motor batteries are in the rear and you can see the charger for those as well. 

If full charge is 3.6 volts per cell then 10 cells would work at 36 volts but if I recall the voltage drops quickly to 3.2 volts per cell under load, leaving me at 32 volts for the majority of the run time.